Multiplanar EMI shielding gasket and method of making

ABSTRACT

A multiplanar, conductive gasket material having a foam core, a conductive web layer comprising a blended mixture of conductive fibers and nonconductive fibers, and a plurality of blended mixture fibers interspersed through the foam core. The blended mixture fibers are heat set within the gasket material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an EMI shielding gasket.Particularly, the present invention relates to an EMI shielding gaskethaving electrical conductivity through the gasket.

2. Description of the Prior Art

EMI shielding gaskets are used to electrically seal gaps in metallicenclosures that encompass electronic components. The gaps between thepanels, hatches, etc., and a housing provide an undesired opportunityfor EMI/RFI to pass through the shield. The gaps also interfere withelectrical currents running along the surfaces of the housing fromEMI/RFI energy, which is absorbed and is being conducted to ground. Thegaps reduce the efficiency of the ground conduction path and may evenresult in the shield becoming a secondary source of EMI/RFI leakage.

Various configurations of gaskets have been developed over the years toclose the gaps to effect the least possible disturbance of the groundconduction currents. Each seeks to establish as continuous anelectrically conductive path as possible across the gaps. Some areuseful in only static applications while others may be used in bothstatic and dynamic applications. A static application is one where partsfunction at a fixed height and where loading force is constant. Adynamic application is one where parts function under a varying heightfrom maximum to minimum limits and where loading forces will varyinversely proportional to height. An example of a dynamic, applicationis one where plates, hatches, etc., are repeatedly separated andreconnected to a housing.

Enclosures that house various electronic components must oftentimes beopened and closed in order to service the electronic components inside.To withstand numerous enclosure openings and closings, an EMI shieldinggasket must be suitable for dynamic applications. Unfortunately, thereare inevitable compromises between the ability of a gasket to smoothlyand thoroughly engage and conform to the surface of the housing adjacentthe gaps, the conductive capacity of the gasket, the ease of mountingthe gasket, the ability of the gasket to withstand abrasive wear andtear as well as repeated compression and relaxation, and the cost ofmanufacturing the gasket. Numerous prior art EMI shielding gaskets havebeen disclosed.

U.S. Pat. No. 6,309,742 B1 (2002, Clupper et al.) discloses an EMI/RFIshielding gasket. The electrically conductive gasket has a metallized,open-celled foam substrate with a skeletal structure and a metal coatingdeposited onto the skeletal structure. The gasket is both recoverableand substantially deformable under low pressure. The metallization ofthe foam is in the form of metallic coating on the skeletal structure ofthe foam. The metallic coating is deposited on the majority of surfacesthroughout the open-celled foam substrate on the skeletal structure. Adisadvantage of the Clupper device is that the metallization processmust be carefully controlled in order to sufficiently coat the gasketwith metal throughout the foam substrate to provide properthrough-conductivity, yet not overly coat with metal such that themetallized foam becomes difficult to compress and/or insufficientlyresilient.

U.S. Pat. No. 6,395,402 B1, (2002, Lambert et al.) discloses methods ofpreparing an electrically conductive polymeric foam. The methods includethe steps of (a) contacting the polymeric foam with a surfactantsolution; (b) contacting the polymeric foam with a sensitizing solution;(c) contacting the polymeric foam with an activation solution; and (d)forming at last one metallic layer on the polymeric foam with anelectroless plating process.

A Laird Technologies new product bulletin discloses a flame retardantconductive foam that provides x, y and z-axis conductivity to enhancethe shielding effectiveness. A disadvantage of the Laird conductive foamis that it is designed for non-dynamic, low stress areas such asinput/output shielding as well as other standard connectorconfigurations.

U.S. Pat. No. 6,465,731 (2002, Stanley Miska) discloses a throughconductive EMI shielding. The Miska device uses a conductive core havingeither metal-plated fibers embedded into the core or a metal-plated foamcore. A disadvantage of this device is that the metal-plated coating canbreak with repeated compression and relaxation causing a degradation inthe through conductivity of the gasket.

The metallic-plated conductive foams of the above disclosures are notre-usable in dynamic applications because the plated metal surfaces thathave been laid within the cells of the foam are rigid. These rigid metalsurfaces will break down after initial compression has occurred.

Therefore, what is needed is an EMI shielding gasket material that ispliable and non-deformable for use in dynamic applications. What isfurther needed is an EMI shielding gasket material that providesconductance throughout a foam core yet does not have a rigidmetallic-coated composition. What is still further needed is an EMIshielding gasket material that provides conductance throughout the X, Y,and Z-axes. What is still further needed is a method of making an EMIshielding gasket that is inexpensive and retains the foam core'sresilient and compliant characteristics.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an EMI shieldinggasket useful in dynamic applications. It is a further object of thepresent invention to provide an EMI shielding gasket that retains it's arelatively high conductance during the compression and relaxation cyclesof re-use. It is yet a further object of the present invention toprovide substantially similar shielding effectiveness with re-use. It isanother object of the present invention to provide an EMI shieldinggasket that shields in the three-dimensional X, Y, and Z-axes. It isstill a further object of the present invention to provide a method ofmaking an EMI shielding gasket that is inexpensive and retains the foamcore's resilient and compliant characteristics.

The present invention achieves these and other objectives by providing amultiplanar EMI shielding gasket that has a flexible foam core, aconductive fiber web on at least one side of the foam core, and aplurality of blended conductive fibers interwoven throughout the foamcore. The plurality of blended conductive fibers extends from the topsurface of the foam core, through the interior of the foam core, andprotrudes outward from the bottom surface of the foam core. The EMIshielding gasket exhibits X, Y, and Z axis conductivity due to thepresence of the blended conductive fibers above, below, and throughoutthe foam core, respectively.

The foam core of the multiplanar EMI shielding gasket is composed of aconventional polymeric flexible cellular foam. The foam core may beopen-celled, partially open-celled, or close-celled, depending upon theneeds of the specific application. Conventional polymeric flexiblecellular foams include but are not limited to thermoplastic elastomer(TPE) such as SANTOPRENE®, NEOPRENE® or a polyurethane-containingmaterial such as polyester, polyether, polyurethane, or combinationsthereof. The foams preferably have a thickness ranging from about 0.5 toabout 50 millimeters.

The conductive fiber web of the multiplanar EMI shielding gasket iscomposed of a homogenously blended mixture of a plurality of conductiveand nonconductive fibers. The conductive fibers of the fiber web aretypically composed of silver, silver/copper, or silver/nickel on a nylonstaple fiber of 1 to 15 Denier in size and 1 to 5 inches in length. Thenon-conductive fibers of the fiber web are typically composed ofbi-component polyester fibers of 1 to 15 Denier in size and 1 to 5inches in length. The conductive and non-conductive fibers of thepresent invention are blended in a typical ratio of 75/25, with thisratio adjustable upward or downward, depending upon the conductivity andshielding effectiveness (SE) of the desired end product.

The conductive fiber web is formed by blending the conductive andnon-conductive fibers into a homogeneous mix and then feeding theblended fibers into a textile carding machine or a randomizing fiberwebber. This process produces a 40 to 80 inch wide web with a weightbetween 10 to 200 grams per square yard, depending on the desiredconductivity and shielding effectiveness of the finished product.

A stiffening fabric may optionally be added between the foam core andthe conductive fiber web to create a stiffer final EMI gasket material.The stiffening fabric may be necessary when the foam core has athickness of less than 5 millimeters. The stiffening fabric may be usedwhere a finished strip of gasket product having a core thickness of lessthan five millimeters lacks a certain needed firmness, stiffness orrigidity.

In order to enhance both the conductivity and shielding effectiveness ofthe conductive gasket of the present invention, a thin layer of aluminumfoil is inserted between the conductive fiber web and the flexiblepolyurethane foam core prior to the needlepunching operation. Thealuminum foil layer has a thickness preferably between 0.0005 to 0.002inches and is available from Neptco, Inc., Pawtucket, R.I. Addition of athin aluminum foil layer improves the EMI gasket strip or die cutInput/Output gasket shielding performance.

In order to improve the ability of the strip gasket to stand up torepeated cross direction shear action, the entire back surface of thestrip gasket or Input/Output gasket is covered with a specialhoneycomb-pattern, pressure sensitive adhesive. The special honeycombpattern has diamond shaped apertures that allow for connectivity betweenthe conductive back side of the multiplanar gasket and the surface uponwhich it is attached. This technique allows for the use of anon-conductive, less expensive PSA adhesive product such as thatavailable as product number RX650ULT from Scapa North America, Windsor,Conn. The entire gasket or I/O surface is adhered solidly to the cabinetbox or enclosure door providing superior cross shear action.

The multiplanar EMI shielding gasket material is formed by depositingthe conductive fiber web onto the polymeric, flexible, cellular foam.The assembly of the conductive fiber web and the polymeric foam is thenpresented to a needlepunch loom. The loom needlepunches the blendedfibers of the conductive fiber web through the foam. To avoid tearing upthe foam, a special chisel pointed needle is used. The loom needle has achisel point with a plurality of angled barbs along the needle shaftfrom the chisel point for a predefined distance. The barbs typicallyhave a five-degree angle but loom needles having larger angled barbs mayalso be used. The larger the barb angle, the greater the number offibers from the conductive fiber web carried through the foam.

Because the needlepunching process on the needlepunch loom tends tocreate a “carpet pile” like characteristic to the conductive fiber web,the intermediate product is heat-set to lock the fibers into place. Theheat-setting process involves heating the intermediate product to thesoftening point of the bi-component polyester fibers, which is lowerthan the softening point of the conductive fibers. The softening pointof the nonconductive, Bi-component polyester fibers is typically 240 to280 degrees Fahrenheit.

The construction of the present invention provides flexibility andcorner-ability that is unmatched by conventional products. It is theabsence of a stiff, metallized woven fabric as well as the absence oflayers of coatings or adhesives used to adhere the foam to theconductive fabric that allows the present invention to closely contactthe electronic enclosure in order to give as close to perfect an EMIseal as possible; a critical characteristic needed for future high clockspeeds with ultra-fine electric wave amplitude.

Further, the simplicity of manufacturing and the lower cost of the rawmaterials used provides important savings in the cost of the finishedproduct. In addition, the simplified manufacturing process provides auser with a much wider or broader range of applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged, perspective view of the present invention showingthe multiplanar EMI gasket material.

FIG. 2 is a cross-sectional view of one embodiment of the presentinvention showing a partially formed sheet of EMI gasket material.

FIG. 3 is a cross-sectional view of a second embodiment of the presentinvention showing a partially formed sheet of EMI gasket material.

FIG. 4 is a cross-sectional view of a third embodiment of the presentinvention showing a partially formed sheet of EMI gasket material havinga stiffening fabric added between the foam core and the conductive fiberweb.

FIG. 5 is a cross-sectional view of a variety of thermoformed shapesusing the gasket material of the present invention.

FIGS. 6A and 6B are side views of needles used in the needleloomingprocess of forming the multiplanar EMI gasket material of the presentinvention.

FIG. 7 is a side view of a thermoforming line to create the finalmultiplanar EMI gasket material of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments of the present invention are illustrated inFIGS. 1–7. FIG. 1 is a perspective view of a sheet of multiplanar EMIgasket material 100. Multiplanar EMI gasket material 100 has a polymericfoam core 20, a layer of a conductive fiber web 40 on at least one sideof foam core 20, and a plurality of conductive fibers 43 interspersedentirely through foam core 20. Polymeric foam core 20 has a top surface22 and a bottom surface 24 with a plurality of interspersed pores (notshown) located throughout foam core 20. The plurality of interspersedfibers 43 protrude from the bottom surface 24 of foam core 20 andmaintain electrical continuity with conductive fiber web 40. An exampleof an acceptable material for use as foam core 20 is Foamex Ether foamtype YCC240-115 available from Foamex International, East Rutherford,N.J.

Conductive fiber web 40 is a blended mixture of a plurality ofconductive and nonconductive fibers. The conductive fibers and thenonconductive fibers are blended into a homogeneous mix and fed into atextile carding machine or a randomizing fiber webber. The processproduces a forty to eighty inch wide web with a weight between about 10to about 200 grams per square yard. The weight of the web depends on theconductivity and shielding effectiveness desired.

The conductive fibers are typically composed of silver, silver/copper,or silver/nickel on nylon staple fibers of one to fifteen Denier in sizeand one to five inches in length. The nonconductive fibers arenonconductive, Bi-component Polyester fibers available from SteinLimited, Albany, N.Y. A typical blend ratio of the conductive andnonconductive fibers is about 3 to 1 (75/25). The blend ratio, however,may be adjusted depending on the conductivity and shieldingeffectiveness requirements of the end product.

FIG. 2 is a cross-sectional view of the needleloom process of making oneembodiment of EMI gasket material 100. Conductive fiber web 40 isbrought into a layered relationship with foam core 20 forming amultiplanar assembly 50. Multiplanar assembly 50 is then subjected to aneedleloom 500 that processes the multiplanar assembly 50 much like theprocess used for making carpets. Needleloom 500 intersperses individualfibers 43 of web 40 from web 40 through foam core 20 so that a pluralityof individual fibers 43 protrude out of bottom surface 24. Theneedleloom process interlocks, or binds, fiber web 40 to foam core 20forming multiplanar core 90 without the need of an adhesive.

Turning now to FIG. 3, there is illustrated a cross-sectional view ofthe needleloom process making a second embodiment of EMI gasket material100. As described above, conductive fiber web 40 is brought into alayered relationship with foam core 20 forming a multiplanar assembly50. Multiplanar assembly 50 is then subjected to a needleloom 500 thatprocesses the multiplanar assembly 50. Needleloom 500 interspersesindividual fibers 43 of web 40 from web 40 through foam core 20 so thata plurality of individual fibers 43 protrude out of bottom surface 24.The needleloom process interlocks, or binds, fiber web 40 to foam core20 forming multiplanar core 90. A second conductive fiber web 40′ isthen brought into a layered relationship with foam core 20 at bottomsurface 24 forming a multiplanar assembly 50′ composed of multiplanarcore 90 and second conductive fiber web 40′. Multiplanar assembly 50′ isthen subjected to a second needleloom 500′ that processes themultiplanar assembly 50′. Needleloom 500′ intersperses individual fibers43′ of web 40′ through foam core 20 so that a plurality of individualfibers 43′ protrude and make contact with conductive web 40. The secondneedleloom process interlocks fiber web 40′ to multiplanar core 90forming multiplanar core 90′.

In instances where the thickness of foam core 20 is too thin such as,for example, less than five millimeters in thickness, the gasketmaterial 100 may lack a certain needed firmness, stiffness or rigidity.FIG. 4 is a cross-sectional view of the needleloom process making athird embodiment of EMI gasket material 100. A non-conductive,stiffening fabric 30 is brought into a layered relationship betweenconductive fiber web 40 and foam core 20 forming a multiplanar assembly150. Multiplanar assembly 150 is then subjected to a needleloom 500 thatprocesses the multiplanar assembly 150. Needleloom 500 interspersesindividual fibers 43 of web 40 from web 40 through stiffening fabric 30and foam core 20 so that a plurality of individual fibers 43 protrudeout of bottom surface 24. The needleloom process interlocks, or binds,fiber web 40 to stiffening fabric 30 and foam core 20 formingmultiplanar core 190.

Stiffening fabric 30 is preferably a web of nonconductive fibers havinga blend of about 4 to 1 (80/20) of Bi-component polyester and regularpolyester fibers, respectively. Bi-component polyester typically has asoftening point of about 240° F. to 280° F. An added advantage ofincorporating stiffening fabric 30 is to utilize the stiffening fabric'sthermoforming characteristics in various three-dimensionalcross-sections that provide better shielding strip and input/outputgaskets. FIG. 5 provides illustrative examples of various shapes, butshould not be construed as limited to only those illustrated shapes.Each of the shapes include a foam core 20, a stiffener fabric 30, aconductive fiber web 40, and a plurality of conductive fibers 43penetrating through foam core 30, all as previously disclosed.

To enhance both the conductivity and shielding effectiveness of gasketmaterial 100, an aluminum foil layer may be inserted between conductivefiber web 40 and foam core 20. FIG. 4 may be used to illustrate theenhanced gasket material 100 as the stiffening fabric 30 is simplyreplaced by the aluminum foil layer. The remaining needlepunchingoperation is the same. It is noted that a combination of a stiffeningfabric 30 and an aluminum foil layer may also be used. In such a case,the aluminum foil layer will preferably be adjacent the stiffeningfabric 30.

FIGS. 6A and 6B are side views of two illustrative examples of needlesused in needleloom 500. FIGS. 6A and 6B illustrate needlepunch needles510 a and 510 b. Needles 510 a and 510 b have identical crank portions512 a, 512 b, shank portions 514 a, 514 b, taper portions 516 a, 516 b,barb portions 518 a, 518 b, and chisel point portions 520 a and 520 b.Barb portion 518 a of needle 510 a has a plurality of five degree angledbarbs 519 a. Barb portion 518 b of needle 510 b has a plurality oftwenty degree angled barbs 519 b. The greater the degree angle on thebarbs, the greater the number of fibers 43 carried by the needle throughfoam core 20. Needles 510 a, 510 b can be purchased from Foster NeedlesCompany and are type 15×18×40×3 CBA F56-3B/CP special 6 barb needles.Particularly important features of needles 510 a, 510 b are that theneedles (a) permit penetration of a cross-section of the foam withouttearing it, and (b) carry the individual fibers 43 through the foam core20 to the opposite side.

Another important feature of the present invention is the use of a fiberblend of conductive fibers and nonconductive, lower melting pointfibers. The nonconductive, lower melting point fibers are Bi-componentpolyester (as previously disclosed) and have a softening point of about240° F. to 280° F. Although the multiplanar cores 90 and 90′ have thenecessary qualities of a conductive gasket material, the fibers in themultiplanar cores 90 and 90′ tend to create conductive fiber lint. Thisconductive fiber lint is unwanted, particularly in electronicapplications where loose conductive fibers may cause unintended shortingin the electronic circuits. To prevent this from occurring, themultiplanar cores 90 and 90′ are subjected to a heat treatment process.

Turning now to FIG. 7 there is shown an illustrative example of such aheat treatment process. Thermoforming process 700 includes a roll ofmultiplanar core 90 or 90′, a heated chamber 720, a cold shaping die730, puller belts 740, and a cutting mechanism 750. Multiplanar core 90or 90′ is unrolled and fed over a roller 710 into heated chamber 720.Heated chamber 720 includes heated rollers such as a textile calendar ortwo heated belts that brings the multiplanar core 90 or 90′ to thesoftening point of the nonconductive, Bi-component polyester fibers,which is typically about 240° F. to 280° F. This heating process locksthe conductive fibers in place and avoids any subsequent movement orloss of conductive fibers while maintaining the gasket's resilient andcompliant characteristics. The resultant material forms a multiplanarEMI gasket material that is conductive not only in the X and Y axis butalso in the Z axis through the gasket.

At this point, the gasket material 100 may be stored for later diecutting or may be die cut after passing through heated chamber 720. Ifdie cutting is performed as part of the thermoforming process 700, thenthe gasket material 100 is cooled to about ambient temperature beforepassing through cold shaping die 730. Cold shaping die 730 stamps orcuts the required template into gasket material 100, is passed throughpuller belts 740 and moved to the cutting mechanism 750 where thedie-cut gaskets are cut to length.

The present invention and method provides a continuous sheath offlexible cellular foam having the ability to conduct electricity and toprovide EMI shielding in a 3-axis (X-Y-Z) configuration. The presentinvention is the basic conductive gasket material for the manufacture ofstrip gaskets in very small and very wide widths as well as die cutInput/Output gaskets for very small and very large surface areas. Itshould be understood that the composite of conductive fibersneedlepunched through the foam may be made in a variety of fiberdensities and foam thickness that vary depending on the application. Theconductive gasket material of the present invention may also be slitinto individual strips for making continuous strip gaskets. Further, asexplained above, using a layer of stiffening fabric made of theBi-component polyester provides the ability to create a variety ofthermomolded gasket shapes.

Although the preferred embodiments of the present invention have beendescribed herein, the above description is merely illustrative. Furthermodification of the invention herein disclosed will occur to thoseskilled in the respective arts and all such modifications are deemed tobe within the scope of the invention as defined by the appended claims.

1. A multiplanar electromagnetic interference (EMI) conductive gasketmaterial comprising: a foam core; at least one conductive web layercomprising a blended mixture of a plurality of conductive andnonconductive fibers disposed on a first side of said foam core; and apredefined quantity of said blended mixture of said plurality ofconductive and nonconductive fibers extending completely through saidfoam core to a second side of said foam core.
 2. The conductive gasketmaterial of claim 1 further comprising a nonconductive stiffener fabricdisposed between said at least one conductive web layer and said foamcore.
 3. The conductive gasket material of claim 2 wherein saidplurality of conductive and nonconductive fibers are locked intoposition by said plurality of nonconductive fibers.
 4. The conductivegasket material of claim 3 wherein said plurality of nonconductivefibers are made of bi-component polyester.
 5. The conductive gasketmaterial of claim 2 wherein said stiffener fabric is a web ofnonconductive fibers having a blend of bi-component polyester andregular polyester fibers.
 6. The conductive gasket material of claim 1further comprising a second conductive web layer disposed on said secondside of said foam core.
 7. The conductive gasket material of claim 1wherein said plurality of conductive fibers of said blended mixture ofsaid plurality of conductive and nonconductive fibers are locked intoposition by said plurality of nonconductive fibers.
 8. The conductivegasket material of claim 1 wherein said at least one conductive webmaterial has a blended ratio in the range of about 1 to 1 to about 3 to1 of said conductive fibers to said nonconductive fibers.
 9. Theconductive gasket material of claim 1 wherein said at least oneconductive web material has a blend ratio of at least 1 to 1 of saidconductive fibers to said nonconductive fibers.
 10. The conductivegasket material of claim 1 wherein said plurality of nonconductivefibers are made of a bi-component polyester having a softening point ofabout 240° C. to about 280° C.
 11. The conductive gasket material ofclaim 1 further comprising an aluminum foil layer disposed between saidat least one conductive web layer and said foam core.